350 MACTRID^. 



nectiug cartilage and small external ligament completing 

 the union ; the inner surface of the valves invariably pre- 

 sents a considerable pallial sinus. The animals have their 

 mantles variously open, often with the margins united for 

 a considerable distance in front ; the siphons are united 

 and fringed with simple cirrhi at their orifices. 



MACTRA, LiNN.'Eus. 



Shell more or less triangular, solid or thin, equivalve, 

 more or less inequilateral, sometimes nearly equilateral, 

 slightly gaping at the extremities ; surface smooth or trans- 

 versely striated, invested with a striated epidermis ; mus- 

 cular impressions rounded or oblong ; pallial sinus shallow, 

 but wdde ; hinge composed of a V-shaped cardinal tooth 

 in one valve, locking into a marginated pit in the other, 

 and a long lateral tooth on each side of the same valve 

 which like the primary one is lodged in a deep groove with 

 tooth-like margins in the other ; cartilage pit triangular, 

 a small external ligament immediately behind it. 



Animal triangular or oblong ; its mantle freely open in 

 front as far as the siphons, the margins more or less dis- 

 tinctly fringed ; the siphons are united to their extremities, 

 which are surrounded with fringes of simple cirrhi ; the 

 foot is strong, changeable in shape, linguiform, and genicu- 

 lated ; the labial tentacles are long and pointed, pecti- 

 nated on their inner sides ; the outermost branchial leaflet 

 in each pair is shorter than the other. 



The MactrtE are found on sandy coasts at various depths, 

 though the majority of species, and especially the larger 

 kinds, are littoral. Shells of this genus are often cast on 

 shore by the waves. The animals live buried in sand at a 

 small depth beneath the surface, and arc active and power- 



